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Melting Techniques

There are a variety of methods used in foundries for melting the metal for casting.  The selection of an appropriate technique is dependant on factors such as the metal being melted, the amount of molten metal required for the production run and the area available to house the melting equipment.  The range of melting equipment typically used can be categorised as follows:

  • Electric Arc Furnaces - This is the principal type of furnace used in the steelmaking proces.  The constituents of the required alloy are loaded (or 'charged') into the furnace, where carbon electrodes strike an arc on to the charge during the melt down phase.  The furnace can also be used to hold the molten metal while it is being refined.  The molten metal is removed from the furnace by tilting it and pouring the metal into one or more ladles for the casting operation.
  • Induction Furnaces -  A water cooled copper coil carrying alternating current produces a magnetic field, which in tern induces eddy currents in the charge.  These eddy currents heat and evenutally melt the metal.  A second magnetic field is generated by the induced current in the charge.  The opposed magnetic fields result in a mechanical force, which stirs the molten metal.  The benefits of this stirring include the production of a thermally and chemically homgeneous melt and excellent alloy and charge absorption.
  • Vacuum induction Melting - Induction melting can be performed within a vacuum if exotic, highly reactive and high purity metals are being melted.  It has another of additional advantages, including low losses of alloying elements due to oxidation, achievement of close compostional tolerances and removal of dissolved gases.
  • Reverberatory or Hearth Furnaces - use a fuel source (gas, coke, etc.) to heat the entire furnace with the heat radiating from the walls and ceiling used to melt the charge, although systems whereby roof and ceiling burners are aimed directly at the metal surface are now more common.  The molten metal is removed from the furnace through a tap hole, or through a pouring spout it the case of tilting reverberatory furnaces.
  • Crucible Furnaces - The crucible furnace is very versatile and flexible.  Crucibles are vessels made out of refractory material, steel of cast iron, that are heated inside a furnace (either fuel fired or induction) such that the charge doesn't come into contact with the flame and flue gases (where present).  The charge is heated by heat conducted through the crucible walls.  Molten metal can either be dipped out with hand ladles, poured directly from a removable crucible, or poured out of a tilting furnace containing the crucible.
  • Cupolas - In its simplest form, a cupola is a cylindrical shaft that burns coke with the heat intensified by the blowing of air through a series of nozzles, or tuyeres.  The charge is put into the furnace through a charge door at the top of the cupola, along with replacement coke.  The metal is melted by the hot gases from the coke combustion, and the molten metal collects in a well at the base of the cupola.  This metal is either continuously of intermittently tapped off through a tap hole.
  • Vacuum Arc Skull Melting and Casting - All operations take place within a vacuum tight chamber.  A titanium or titanium alloy electrode is lowered into a water cooled copper crucible. An arc is struck between the electrode and the crucible, and since the crucible is water cooled, a solidified titanium skull forms on the crucible surface.  This avoids direct contact between the melt and the crucible.  When the required amount of liquid titanium is held in the crucible, it is poured into the casting mould, also within the vacuum chamber.